TL;DR
Mercury has shrunk by four to six miles of radius since it formed, and earlier work underestimated the contraction by 10 to 30 percent because impact debris was hiding the planet's wrinkles.
Mercury is the smallest planet in the solar system, with a surface area roughly the size of the Indian Ocean. But at birth it was much bigger, its radius possibly four to six miles longer than today's.
The planet's wrinkles were covered up
A planet contracts as it cools, like dough going cold. That contraction leaves rocky wrinkles on the surface, which scientists call shortening structures.
Models predicted those wrinkles would be spread evenly. New maps show they are not. The reason: ejecta, the debris thrown out by asteroid impacts, is sitting on top of a lot of them.
So earlier estimates came in low. The new correction: contraction has been underestimated by 10 to 30 percent.
Someone is going to look in November
The correction matters for how Mercury's interior evolved. BepiColombo, a joint European-Japanese mission, is due to slip into orbit around the planet this November and check it directly.
Mercury is the only planet in the solar system outranked by moons — Jupiter's Ganymede and Saturn's Titan are both bigger. The secrets it is holding, though, are not small.
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